1. Field of the Invention
The present invention relates to rigid polymeric substrates based on thermotropic polymers well adapted for use as magnetic recording media, and, in particular, to magnetic recording discs produced therefrom.
2. Description of the Prior Art
As is well known to this art, magnetic recording discs include a circular, rigid substrate which is smooth and thin (thickness on the order of 1 to 2 mm), which generally has a coaxial opening in the center thereof, adapted to receive appropriate holding and driving means, and with at least one of its face surfaces comprising a suitable magnetic coating of very low thickness (for example, 0.3 to 0.5 .mu.m in the case of the magnetic layers based on magnetic pigments or 0.05 to 0.2 .mu.m in the case of discs provided with a thin metal surface layer).
Surmounting the magnetic coating, one or more magnetic read-write heads are arranged in a manner known per se and, when the disc is rotated (speed of rotation on the order of 3,600 revolutions/minute), these "hover" at a height on the order of 0.2 to 0.6 .mu.m above the surface of the disc.
To ensure that the position of the head relative to the disc is perfectly stable, the substrate defining a part of this disc's construction has to meet a certain number of requirements, the principal among which are the following:
(a) a high rigidity (reflected particularly in a high modulus of elasticity) to avoid the phenomenon of buckling under the influence of stresses; PA0 (b) an excellent surface quality (reflected particularly in low roughness); PA0 (c) a high dimensional stability with regard to temperature or moisture (reflected particularly in a low coefficient of thermal expansion in the radial direction); PA0 (d) a low moment of inertia (reflected particularly in a low value of relative density); and PA0 (c) good chemical and thermal behavior (reflected particularly in a high value of the deformation temperature under load (abbreviated to: DTUL), in order that, when the magnetic coating deposition operations are carried out, it readily withstands, on the one hand, contact with the solvents or solvent mixtures which are employed when a dispersion of magnetic particles are to be deposited (in the case of magnetic layers based on magnetic pigments) and, on the other hand, the high temperatures reached either at the time of deposition (in the case of thin metal layers), or at the time of crosslinking of the binders (in the case of layers based on magnetic pigments). PA0 (i) a high modulus of elasticity in the radial direction, preferably well above 5,000 MPa (measured at an EH of 50% and at 25.degree. C. according to French Standard NF T51034); PA0 (ii) a low coefficient of thermal expansion in the radial direction, comparable or even lower than that corresponding to aluminum, which is on the order of 30 .mu.m/m/.degree.C.; PA0 (iii) a high DTUL of at least 150.degree. C., in order, for example, to enable the coated disc to be readily transferred through a heated oven with the objective of crosslinking the binder present in the magnetizable material which is deposited thereon; PA0 (iv) a very low and preferably nonexistent shrinkage on demolding; PA0 (v) a good resistance to chemical agents in order that the polymeric substrate is not attacked, for example, by the solvents which may be used in the preparation of the liquid dispersion containing the magnetizable material which is deposited onto the support; and PA0 (vi) resistance to change due to atmospheric agents, such as moisture, under service conditions which otherwise could result in dimensional changes which are equal to or greater than those effected by a temperature differential.
Aluminum alloys such as, for example, alloys based on aluminum (96% by weight) and magnesium (4% by weight) of the AA 5086 type meet these various requirements relatively well. They are, however, materials which are costly to machine or to polish. Furthermore, iron-rich oxidizable intermetallic compounds are generally present within these materials, giving rise to the formation of segregated regions, the size of which ranges from 2 to 10 .mu.m, and which behave differently either over the course of the machining and polishing, or during aging.
Glass can also constitute a material which is relatively well adapted as a magnetic coating substrate for a recording disc. However, the disadvantages which are attributed to glass in the majority of cases are as follows: firstly, glass is brittle (in materials science, brittleness typically connotes the absence of plasticity and abrupt failure upon impacts); furthermore, glass is costly to work and to polish; and, lastly, problems may also arise in respect of the adhesion of the magnetic coating which it is to receive.
Because of the high price of the aluminum substrate to the overall cost of manufacture of a rigid magnetic disc (on the order of 30%), as well as the additional cost associated with the need to protect the surface of the substrate with an anti-corrosion layer (for example, a deposit of electrolytic nickel in the case of a disc provided with a thin magnetic layer, and also because of the many disadvantages associated with glass, it has been necessary to investigate other materials which are of greater interest for the production of rigid magnetic disc substrates. The advent of novel thermoplastic polymers with advantageous properties and the use of injection molding in the audiovisual field have resulted in consideration being given to the replacement of aluminum and glass with an appropriate polymer. These are generally inexpensive materials which are very light and which therefore have a low moment of inertia. However, it is quite difficult to provide the combination of the following desirable properties in the case of the circular substrates fabricated from these polymeric materials:
It should also be noted that difficulties may still be encountered when the polymer is injection molded, if no precautions are taken to initially select a material which is highly fluid in melt form. The difficulties which are encountered using viscous materials may give rise to internal stresses which will be translated into surface defects in the finished substrate and will cause buckling, inherent in thin-walled injection-molded viscous materials.
To date, acceptable polymeric substrates appear to have been produced (cf. the periodical Plastics Technology, April 1985, pages 73 et seq.), using:
(1) polyetherimides such as, for example, the material marketed under the trademark Ultem 1000; however, this polymer has a low rigidity reflected in an elasticity modulus of approximately 3,000 MPa, and a high coefficient of thermal expansion in the radial direction, of approximately 56 .mu.m/m/.degree.C., which makes it incompatible with all of the existing tracking systems for rigid discs comprising an aluminum substrate; and
(2) polycarbonates or acrylic polymers (cf. JA-A-59/231,750), but both of these materials have an excessively limited temperature behavior.